The present invention relates to a liquid crystal display device(hereinafter "LCD"), more particularly to a liquid crystal display of homeotropic alignment mode having multi-domain.
Currently, a twisted nematic mode(hereinafter "TN mode") LCD used in the notebook computers and other display devices, has high optical efficiency and stability in operation.
However, the TN mode LCDs has the intrinsic property of narrow viewing angle.
Consequently, an In-Plane Switching mode(hereinafter "IPS") LCD in which liquid crystal is driven by an electric field being formed parallel to substrates, has been suggested by Hitachi, a corporation of Japan so as to solve the narrow viewing angle characteristics.
However, the IPS mode LCD also has a drawback that a color shift is generated according to the viewing directions, i.e. the azimuth angles.
So as to eliminate the color shift occurrence, an LCD device capable of maintaining wide viewing angle and simultaneously eliminating the color shift, has been suggested by the present inventors. (U.S. Ser. No. 09/107,797)
Referring to FIG. 1a, a conventional LCD will be described. An upper substrate 10 and a lower substrate 20 are opposed each other with a selected distance d1(hereinafter "cell gap") and a liquid crystal layer 30 is formed between the two substrates 10 and 20. A first driving electrode 21 and a second driving electrode 22, the two electrodes 21,22 for driving molecules 30A in the liquid crystal layer 30 (hereinafter "LC molecules") are formed at the lower substrate 20. Both electrodes 21 and 22 are arranged parallel each other in an x direction shown in the drawings.
A third driving electrode 11 and a fourth driving electrode 12 are formed at one side of the upper substrate 10 which faces the liquid crystal, both electrodes 11 and 12 are formed in parallel each other in a y direction which is substantially perpendicular to the x direction. Herein, a distance l1 between the first driving electrode 21 and the second driving electrode 22, and a distance l2 between the third driving electrode 11 and the fourth driving electrode 12 are equal or almost equal to each other. Further the distances l1 and l2 are designed to be larger than the cell gap d1.
The LC molecules of the liquid crystal layer 30 has a positive dielectric anisotropy. Long axes of the respective LC molecules are arranged in parallel to the electric field direction when an electric field is applied.
Alignment layers(not shown) for determining the initial aligning state of the LC molecules 30A are formed at an inner surface of the upper and the lower substrates 10 and 20. Herein, the alignment layer is a homeotropic layer at a pretilt angle of 85.degree.-90.degree..
A polarizer 23 is attached to an outer surface of the lower substrate 20 and an analyzer 13 is attached to an outer surface of the upper substrate 10. Polarizing axis of the polarizer 23 is arranged in a "P" direction which is deviated from the electric field generated between the first and the second electrodes by approximately 45.degree. preferably, and polarizing axis of the analyzer 13 is arranged in an "A" direction which is perpendicular to the "P" direction.
In order to compensate the refractive anisotropy of LC molecules, a phase compensation film 14 is sandwiched between the upper substrate 10 and the analyzer 13.
As shown in FIG. 1a, when no electric field is formed at the electrodes 11,12,21,22 of the above described arrangements, long axes of the respective LC molecules are arranged perpendicular to the upper substrate 10 and the lower substrate 20 according to the influence of the homeotropic alignment layer (not shown). Consequently, an incident light to pass the polarizer 23, hardly passes the analyzer 13 since said light does not change its polarizing state while passing the liquid crystal layer 30 and therefore the screen becomes dark. At this time, the refractive anisotropy of LC molecules 30A is compensated by the phase compensation film 14 and then a complete dark state is achieved at all points in the screen.
On the other hand, when a voltage is applied to the electrodes 11,12,21,22, an electric field F1 is formed between the first and the second electrodes 11 and 12, and an electric field F2 is formed between the third and the fourth electrodes 21 and 22. At this time, the respective electric fields F1 and F2 are in parallel with surfaces of the substrates 10 and 20 at regions adjacent to the substrates, and the electric fields become an elliptic shape as they recede from the substrates 10 and 20, i.e. as they approach a middle layer of the liquid crystal layer 30. Accordingly, the entire structure of the electric fields F1 and F2 has the shape of an ellipse being twisted by 90.degree. with respect to the middle layer. Herein, the cell gap d1 should be larger than the distances l1 and l2 between the electrodes, and therefore electric fields F1, F2 in parallel to the substrates 10,12 are formed respectively.
The LC molecules 30A arranged perpendicular to the substrates are tilted in the form of the electric fields F1 and F2, and a light leakage is occurred. Herein, the LC molecules 30A are arranged symmetrically in every direction according to the 90.degree. twisted electric-fields, and therefore a quadruple domain is formed without incurring additional rubbing process. The quadruple domain prevents the color shift occurrence and improves the viewing angle characteristics in LCDs.
However, the conventional LCD as described above still has following problems.
Generally, when the LCDs are set in horizontal level, their molecules are arranged symmetrically and no color shift is occurred at every azimuth angle. However when they are set obliquely, the color shift occurs.
More detailed description is given with reference to FIG. 2. When the LCD device is set obliquely, a viewer regards the entire electric field as set obliquely. Consequently, the arrangement of LC molecules seems to be asymmetric and the cell gap d2 seems to be enlarged. Furthermore, factors of the refractive anisotropy .DELTA.n and cell gap d, both determining the color shift are variable and, which is resulted in the color shift occurrence.
In addition, the cell gap is enlarged since the LCD device is set obliquely and response time thereof is degraded. Therefore, the threshold voltage of the LCD device is increased according to the following equation 1. EQU Vth=.pi.l/d(K2/.epsilon..sub.0 .DELTA..epsilon.).sup.1/2 equation 1
wherein,
Vth indicates threshold voltage, PA1 l indicates distance between electrodes, PA1 d indicates cell gap, PA1 K2 indicates twist elastic coefficient, PA1 .epsilon..sub.0 indicates dielectric constant, and PA1 .DELTA..sub..epsilon. indicates dielectric anisotropy.
As shown in the equation 1, the driving voltage of LCD device varies according to the value of l/d. As the cell gap increases, great quantity of driving voltage is required.